The present invention relates generally to metal-organic frameworks, and more specifically to such frameworks, which are reticulated into a predetermined porous material with a tailored pore size and/or functionality, and their application as sorbents for storing gases such as methane.
One of the great challenges in porous materials is the design and the achievement of a desired porous material with a tailored pore size and/or functionality. To date, it has not been possible to consistently and efficiently (with a high yield) render porous materials having predetermined characteristics.
A particular goal is to alter chemical composition, functionality, and molecular dimensions without changing the underlying topology. See A. Stein, S. W. Keller and T. E. Mallouk, Science 259, 1558 (1993); and P. J. Fagan and M. D. Ward, Sci. Am. 267, 48 (1992). Although this has been a dream of scientists and engineers for most of the last century, little progress has been achieved largely due to lack of control over the course of molecular assembly and the inability to predict the orientation of atomic groups in crystals. Unlike the process of building organic molecules, where it is possible to execute the total synthesis of complex ring systems in a step-by-step fashion, the insolubility of extended solids generally necessitates that their assembly be accomplished in a single step. See O. M. Yaghi, M. O'Keeffe, and M. Kanatzidis, J. Solid State Chem. 152, 1 (2000).
Porous materials are mainly used for gas/liquid separation, catalysis, luminescence-based sensors, and gas storage. To achieve a specific application, a porous material with a defined pore size and function is needed. To achieve these challenging objectives, many scientists have devoted their knowledge and programs to develop this area.
A stable, porous metal-organic framework was disclosed recently. See Li, Hailian, Mohamed Eddaoudi, M. O'Keeffe and O. M. Yaghi, “Design and synthesis of an exceptionally stable and highly porous metal-organic framework,” Nature, Vol. 402, pp. 276–279 (18 Nov. 1999). This framework was formed by diffusing triethylamine into a solution of zinc(II) nitrate and H2BDC (benzenedicarboxylic acid) in N,N′-dimethyl-formamide/chlorobenzene. This resulted in the deprotonation of H2BDC and its reaction with Zn2+ ions. The rendered cubic crystals were designated metal-organic framework (MOF)-5 and were found to comprise an extended, porous network having a three-dimensional intersecting channel system with 12.94 Å spacing between centers of adjacent clusters.
The diffusion of base into the solution is generally accepted in the literature as being an important step in the process of fabricating such MOFs. See, for example, Eddaoudi, Mohamed et al., “Modular Chemistry: Secondary Building Units as a Basis for the Design of Highly Porous and Robust Metal-Organic Carboxylate Frameworks,” Accounts of Chemical Research, Vol. 34, No. 4, pp. 319–330 (Web publication date 17 Feb. 2001). This article states that a key step to obtaining crystals is to slowly diffuse an organic amine into the reaction mixture.
Although the MOF-5 crystalline structure described in Nature, supra, has desirable characteristics, the process for making the structure actually renders a mixture of crystalline structures, the MOF-5 being a relatively low percentage of the mix. Further, the Nature MOF-5 structure appears to be limited to a single benzene ring as a linkage between adjacent Zn4(O)O12C6 clusters.
Others have recently pursued the assembly of extended structures from molecular building blocks. See V. A. Russell, C. C. Evans, W. J. Li and M. D. Ward, Science 276, 575 (1997); Y. H. Kiang, G. B. Gardner, S. Lee, Z. T. Xu and E. B. Lobkovsky, J. Am. Chem. Soc. 121, 8204 (1999); and B. F. Hoskins and R. Robson, J. Am. Chem. Soc. 111, 5962 (1989).
Researchers have attempted to formulate frameworks having longer links between adjacent clusters. Synthesis of open frameworks by assembly of metal ions with di-, tri- and poly-topic N-bound organic linkers such as 4,4′-bipyridine has produced many cationic framework structures. However, attempts to evacuate/exchange guests within the pores usually unfortunately results in the collapse of the host framework.
Further, expanded structures have been formed using long links to increase the spacing between vertices in a net, yielding void space proportional to the length of the linker. However, although such expanded structures provide for large pores (and one would therefore expect a high porosity), in practice they are often found to be highly undesirably interpenetrated and to have low porosity.
Thus, it would be desirable to provide a reproducible metal-organic porous material advantageously having a predetermined pore size and function. It would further be desirable to provide such a porous material which desirably retains its topology even with varied linkage compounds. Yet further, it would be desirable to provide a high yielding method for preparing such porous materials. Still further, it would be desirable to provide such a porous material which may advantageously store gases at desirable pressures such as the predominant natural gas methane.